One known pipe coupling of this type is described in Uk-A-1,582,858. The sealing sleeve is of C-shaped longitudinal cross-section having two lips extending axially inwardly towards one another so as to form a groove between the extension and the web portion. When the casing is tightened around the pipe ends the lips of the sealing sleeve are pressed into sealing engagement with the outer surfaces of the pipe ends to form a seal. In use fluid carried by the pipes passes through the gap between the pipe ends into the space between the outside of the pipe end and the inside of the web portion of the sleeve. The fluid enters the grooves between the extensions of the lips causing the sealing pressure of the extensions against the pipes to increase as the pressure of the fluid increases. A disadvantage of such an arrangement is that substantially the whole of the inner surface of the web portion is exposed to the pressure of the fluid from the pipe. Where the fluid pressure is high, this can cause the casing to bulge out at its axial middle portion.
According to the present invention in a first aspect there is provided a pipe coupling for joining together two pipes in a fluid-tight manner, comprising a tubular sealing sleeve and tensioning means for tightening the casing around the sleeve, the sleeve including two sets of annular inwardly projecting sealing ribs on the inner surface of the sleeve which engage the respective pipe ends in a fluid tight manner.
With this arrangement only the portion of the sleeve between the axially innermost pair of ribs is exposed to the hydrostatic pressure of the fluid in the pipe. The casing is therefore less prone to bulging at high hydrostatic pressures and, in consequence, the casing can be made of lighter construction than a conventional coupling with the equivalent performance specification. This means that the coupling is less expensive and easier to handle during manufacture.
Joining together pipes with plain ends with couplings without the need for pipe preparation is quicker and more economical than other methods such as welding, screw threading, flanging, grooving or shouldering. However because of the enormous axial forces that may be involved in pipelines operating at pressures of say 16 bars, it is difficult to construct a coupling that can reliably withstand such forces.
In the coupling of GB-A-1582858, axial restraint is provided by two frustoconical rings with gripping teeth on their inner edges. The gripping rings are located at the sides of the sealing sleeve and held in position within the casing by means of snap rings. When the casing is tightened the gripping teeth bite into the pipe ends to provide axial restraint for the pipes.
There are a number of disadvantages with this arrangement. The gripping teeth are sharp and are exposed. This presents a hazard to the pipe fitters who might cut themselves when handling the couplings prior to fitting.
The teeth are exposed and may come into contact with water and other corrosive liquids depending on the enviroment in which they are employed. The teeth are usually made of a hard steel that is prone to crevice corrosion.
The geometry of the toothed gripping device is upset on clamping on to the pipe. The specially shaped snap rings that are required to support the gripping rings add to the complexity of the coupling and they only provide partial support for the gripping ring. The snap rings may act as fulcrums over which the teeth are bent when loaded and there is thus a potential for the teeth to snap off.
The geometry of the gripping ring is liable to be further upset if a C-shaped sealing sleeve is used for the reasons explained above. The bulging of the casing may cause the toothed gripping ring to be lifted out of contact with the pipe leading to failure.
Another type of known coupling is described in GB-A-2167145. This coupling, does not include toothed rings as described above, but it utilises annular sealing members arranged at either end of an annular sleeve, which has wedge-shaped ends. Two flange members fit at either end of the coupling, and a tensioning means is provided so that the flange members can be urged axially towards one another. This movement compresses the sealing members against the wedge-shaped ends of the sleeve, which in turn forces the sealing members radially inwardly to form a seal against the surface of the pipes.
A plurality of hard gripping members is embedded in each sealing member. Each gripping member has a tooth or teeth formed at one end, and is arranged in the sealing member so that the teeth are flush with, or adjacent, the inner surface of the sealing member. The gripping member is inclined at an angle relative to the axis of the coupling, and the axially outer end of each gripping member is arranged to be completely covered by, but closely adjacent, the outer surface of the sealing member near the corner in the flange member. As the coupling is tightened, the flange members move axially inwardly, compressing the sealing member, and urging the gripping members towards the pipes, causing them to bite into the surfaces of the pipes. Because of the axial displacement between the inner and outer ends of the gripping members, the gripping members will pivot as the sealing member is compressed.
In this coupling, the sealing action of the sealing member against the pipes' surfaces, and the biting action of the gripping members are not independent of each other. The gripping members and the sealing members are acted on simultaneously as the flanges are moved axially. Thus the gripping members and the sealing members restrict one another's movement. If the teeth of the gripping member bite the surface of the pipe before the sealing member is properly seated against the surface of the pipe, this could affect the sealing properties of the seal.
According to the present invention in a second aspect there is provided a pipe coupling for connecting together two pipes in a fluid-tight manner comprising of a tubular casing, a tubular sealing sleeve, and tensioning means for tightening the casing about the sealing sleeve, the sealing sleeve having a circumferential slot in its outer surface and a gripping ring with inwardly projecting gripping teeth located in the slot, the arrangement being such that, in use, tightening the tensioning means causes the sealing sleeve to be pressed into sealing engagement with the pipe ends and the gripping teeth to penetrate the bottom of the slot and to engage the outer surface of the pipe.
In a preferred form of the invention a slot with a gripping ring is provided at each end of the coupling. The slot may be frustoconical with the inner end nearer the axial middle of the coupling than the outer end. The gripping ring is also preferably frustoconical and preferably is an incomplete ring so that it can easily be expanded and compressed.
Preferably the casing has an inwardly projecting flange at each end and the gripping rings are located with their outer edges in the angles between the tubular portion of the casing and the flanges.
The tubular casing preferably comprises an inner casing and an outer casing. The inner casing fits within the outer casing. The inner casing and the outer casing both have longitudinal gaps and the gap in the inner casing is offset circumferentially from the gap in the outer casing so that the sealing sleeve is supported around its entire periphery either by the inner casing, or the outer casing, or both casings.
Both the inner and outer casing may have their axial end margins bent inwardly to form radial flanges.
By locating the gripping rings in slots in the outer surface of the sealing sleeve the gripping teeth are protected and will not injure the pipe fitter when being handled prior to use.
The sealing sleeve may be provided with end seals which form seals between the flanges and the pipe ends. In this way the ingress of water and other contaminants into the seal is prevented, and so the gripping rings are protected from corrosion.
Preferably the slot in the sleeve is wider than the thickness of the gripping rings so that the sealing sleeve does not interfere with the action of the gripping ring. The ring is only restrained by the angle of the inner and outer casings formed between their tubular walls and their annular flanges. The casing bears directly against the outer edge of the gripping ring. When the casing comprises an inner and an outer casing, as described hereinbefore, the inner casing bears against the gripping ring. This further reduces the tendency of the outer casing to bulge, compared with a single piece casing.
According to the present invention in a third aspect there is provided a pipe coupling for coupling together two pipes in a fluid-tight manner, comprising an outer tubular casing, an inner tubular casing located within the outer casing, a tubular sealing sleeve located within the inner casing and means for tensioning the outer casing around the inner casing and the sleeve, the inner and outer casings having inwardly directed end flanges, the tubular sealing sleeve being located between the end flanges of the inner and outer casing, the end flanges of the outer casing being adjacent the end flanges of the inner casing and providing lateral support therefor.
Preferably the coupling includes gripping means for securing the pipes axially, the gripping means being located axially inside the flanges of the inner casing. Preferably the gripping means are in the form of toothed rings. By locating all the components inside the inner and outer casings and their side flanges a strong and stable structure is achieved.
According to the present invention in a fourth aspect there is provided a pipe coupling for coupling together two pipes in a fluid tight manner, comprising an outer tubular casing, a tubular inner casing, a tubular sealing sleeve and means for tightening the outer casing around the inner casing and the sealing sleeve, the inner and outer casing including inwardly projecting end flanges, the sealing sleeve including end seals, so that in use, when the tensioning means are tightened, the sealing sleeve is urged into sealing contact with the outer surfaces of the pipe ends and the end seals form seals between the flanges and the outer surface of the pipe ends. Preferably the end seals are axial extensions of sealing sleeve which are trapped between the flanges and the pipes. The end seals prevent dirt, water or other contaminants entering between the flanges and the pipe and corroding components of the coupling or impairing the effectiveness of the seal.
In the preferred embodiment of the invention, in each aspect, the tensioning means applies a radially compressive force to the casing.
An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, of which: